It is known that different substrates absorb different, specific bands of IR (infrared) radiation. Thus, by directing IR radiation towards a thin film of the substrate it is possible to determine the thickness of the film. By a radiometric analysis it is further possible to determine the proportion of ingredients in the substrate and by continuous monitoring it in possible to establish changes of said proportion. Radiometric analysis is based on the change of energy or absorption of radiation, which in this case has wavelengths within the IR area.
The invention is developed for a printing press but may be used for measurement and control of any thin fluid film. Other applications include but are not limited to: oil films often used in pressing of sheet metal; coatings on paper or metal; and photographic films on substrates within the semiconductor industry.
In a printing press, especially but not exclusively an offset printing press, printing ink is to be supplied to the printing areas of the printing plates, whereas all other areas of the printing plates are to be covered by fountain solution during the entire printing process. The fountain solution is normally a water-based solution. Due to different surface conditions on the printing plates the printing ink and the fountain solution will only adhere to the intended areas. Thus, the printing plates have printing areas and non-printing areas In the printing areas there will be a thin film of an emulsion of fountain solution and printing ink. In the non-printing areas there will be a thin film of fountain solution, which also may include a small portion of ink pigments.
The supply of printing ink is performed by means of inking units (one unit per colour), whereas fountain solution is normally supplied by means of a spray bar or other equivalent device, with a length corresponding to the width of the printing press or the length of the printing cylinder. The spray bar usually contains spray nozzles, but other supply means are also possible. From the spray bar the fountain solution is normally transferred to the printing plates by means of one or more form rollers.
The provided amount of fountain solution and printing ink is normally set to values predetermined for the actual printing situation. However, it is also controlled by the printer, who by an experienced eye supervises the printing plates and sees to it that proper amounts are supplied. However, this manual control lacks proper accuracy and cannot cope with rapid changes in the conditions. Accordingly, the printing result will not be optimal, even if the printer is extremely experienced. It is also common to have a density meter for control of the printing quality.